"passive index investing and instant cash access are fundamentally opposed": I agree, this is the case right now, but we want to change that, because there isn't any fundamental reason for this.
Yes, people will owe tax, but remember that only your gains are getting taxed. So if you get a tax bill, it's because you made money. That is still a net-positive. There are also a lot of things you can do to reduce (though not eliminate) the tax burdon, such as carefully selection what investments to sell, predicting money flow (e.g. not investing a paycheck if rent is due a day later), tax loss harvesting, etc.